FRESH WATER 199 



Vorticella and Epistylis. The base of a submerged stem is some- 

 times surrounded by the fresh water sponge (Spongilla), and splitting 

 up the stems of the reeds usually liberates a crowd of small animals. 



On the upper surface of stones often thickly covered with a 

 growth of moss or alga one may find a wealth of small animals 

 such as insect larvae, and sometimes a stone is quite covered 

 with freshwater sponge. In such a case it is useful to mark a 

 fine specimen and to see it decay away in autumn, the life being 

 continued by multitudes of pinhead-like gemmules which start 

 new sponges in spring. 



One other simple observation may be alluded to. It is usually 

 easy to find some shallow muddy bay by the riverside or in some 

 backwater. On a quiet day, when there is no wind to ruffle the 

 surface, that is a place to watch to see the behaviour of the 

 insects and molluscs and other inhabitants. Very frequently, 

 when one's eye gets accustomed to the scene, one sees on the 

 muddy floor hundreds of little red worms, the thickness of pins, 

 rising an inch or so out of the mud, and swaying backwards 

 and forwards continuously. These are bristle-bearing freshwater 

 worms, distant relatives of the earthworm, and there are many 

 different kinds, Tubifex rivulomm being a common one. It is 

 common where there is some contamination of the water, and 

 thousands may be seen at once. It makes a burrow, a temporary 

 tube in the mud, and it is interesting to see the sudden retreat of 

 the thousands when one strikes the water with a stick. Some of 

 the mud should be taken home and kept for a while in a pie-dish, 

 so that Tubifex may be observed at leisure. In searching the 

 mud with a good lens one may find in midsummer the elliptical 

 grey cocoons out of which the young creep in September. 



TERRESTRIAL 



" Worms " were probably the first backboneless animals to 

 become thoroughly terrestrial. We find this habit in some 

 of the planarians, nematodes, and leeches, but these usually 

 live in very damp places, and it is perhaps to the earth- 

 worms that we should give the credit of discovering the dry 

 land. The great majority of the chaetopod worms are marine ; 



